JPH0778439B2 - How to measure the velocity of a fluid - Google Patents

Abstract

A mass flowmeter for measuring velocity of a flowing fluid employs a thermal sensor (11). The sensor (11) generates heat when supplied with electrical power. A heat sink (25) is located in the fluid flow a selected distance normal to the line of flow of fluid from the sensor. Electrical power is supplied to the sensor (11) to cause a thermal flux to flow from the sensor (11) to the heat sink (25). The flowing fluid modulates this flux. A circuit (23) measures the temperature of the sensor and computes the velocity of the flowing fluid from that measurement.

Description

FIELD OF THE INVENTION This invention relates generally to devices for measuring fluid flow, and more particularly to thermal fluid flowmeters.
Regarding

In summary of the invention, a mass flow meter that measures the velocity of a flowing fluid uses a temperature sensor. When powered, the sensor produces heat. A heat sink is positioned in the fluid flow, perpendicular to the flowing fluid, directly across the sensor. Power is applied to the sensor, causing a thermal flux from the sensor to the heat sink.
The flowing fluid modulates this bundle. The circuit measures the temperature of the sensor and from that measurement calculates the velocity of the flowing fluid.

Prior Art and Problems to be Solved There are many different types of flow meters for measuring the velocity of a fluid. The term “fluid” as used herein,
Means both liquid and gas flow. One category of flow meters is known as a "heat" flow meter. There are two general types of heat flow meters.

In one form, a flow tube is used with a passage for the flow of fluid to be measured. One or more electric heaters are located in the flow tube or the detection tube. The heater provides heat to the fluid as it flows through the detector tube. The temperature is measured at two different points on the detector tube. The difference in temperature between the upstream and downstream points can be correlated with velocity.

In the second category of thermal mass flowmeter, the heater / temperature sensor is located on the boom and submerged in the flowing fluid stream. The circuit senses the temperature response of the sensor as a function of mass fluid flow rate.

Each of these heat flow meters has in common the fact that the rate of heat flow from the sensor to the fluid is directly proportional to the mass flow rate of the fluid. The accuracy of these conventional thermal mass flow meters is
Limited to a relatively narrow range of flow velocities. At low speeds, accuracy is limited by spurious heat losses due to convection and heat leakage to the environment. For high speeds, accuracy is limited by the finite thermal resistance of one or more sensor elements.

To avoid wide velocity range limitations, a common technique is to use a main tube containing laminar flow elements. These laminar flow elements are, for the desired velocity range to be measured,
It is arranged to produce a certain pressure drop. The pressure difference upstream and downstream of the laminar flow element is proportional to the volumetric flow rate of the fluid to be measured.

The detector tube branches from the main tube and re-enters downstream. The detector tube supports the heating element and the sensor. Much less fluid flow than the main tube flows through the detection tube. The velocity of the flow in the detector tube is measured, which is proportional to the main velocity flowing through the main tube. While this can be done, flow rates outside the design range cannot be accurately measured without modifying the laminar flow element.

U.S. Pat. No. 4,517,838 (May 21, 1985)
In (Sun), an example of heat transfer is presented. In this case, a narrow groove is provided in the detection tube. Heating means are mounted in the narrow channels to measure the fluid flow rate. The small size of the detector tube required by such a narrow groove further limits the high fluid velocity measurement capability of the flowmeter.

In the present invention, the sensor is placed in the main flow of fluid. The sensor can generate heat when power is applied. A heat sink is placed in the fluid flowing directly across the sensor, perpendicular to the direction of fluid flow.

The power supplied to the sensor creates a heat flux that flows from the sensor to a heat sink that absorbs heat. The heat flux is modulated by the flowing fluid. The circuit measures the temperature rise of the sensor and calculates the velocity of the flowing fluid from the measured value.

Referring to FIG. 1, a foil strip 11 combining a heat source and a temperature sensor is shown. The sensor 11 is mounted on the heat insulating material 13. The sensor 11 is preferably of the conventional type commonly used to record temperature. As shown in FIG. 4, it has a thin insulating layer or substrate 15 of a material such as Kapton. The substrate 15 is covered or laminated with a conductive metal layer 17. The metal layer is a wavy coil
Etched in pattern 19. To produce the coil 19, fine very fine wires are put into the metal layer 17. The coil 19 and the substrate 15 have flat surfaces. This common type of sensor is conventionally available.

In the present invention, the coil 19 of the sensor 11 is connected to a power source 21 to radiate heat from the sensor 11, as shown in FIG. Power supply 21 is coil 19 of sensor 11 (Fig. 4)
Provides DC power flowing through. The coil 19 of the sensor 11 produces heat, which is emitted from the sensor. Measuring circuit 23
Measures the power supplied and thus obtains an indication of temperature.

The heat sink 25 is attached directly to the sensor 11.
The heat sink 25 is made of metal and transfers heat easily, thus attracting and absorbing the heat generated by the sensor 11. The surface of the heat sink 25 is flat and the sensor
Parallel to eleven. The fluid flow is perpendicular to the normal of sensor 11 and heat sink 25.

For the purpose of analyzing and describing behavior, sensors 11
An incremental volume 27 of fluid is shown, located within the effective volume 28 between the heat sink 25 and the heat sink 25. The following definitions are provided.

z = distance between sensor 11 and incremental volume 27 dz = thickness of incremental volume 27 A = area of incremental volume 27 T ₀ = temperature of sensor 11 T _a = ambient temperature of fluid flowing through heat sink and flowmeter T = Temperature of fluid in incremental volume 27 Q = heat quantity C = heat capacity or specific heat of fluid (BTU / lb.F) D = density of fluid (lb / cu.in) K = thermal conductivity of fluid (BTU in / hr. sq.ft.F) d = differential operator t = time V = average molecular velocity in feet per minute (fpm) of fluid flowing through sensor 11 W = supplied to fluid by sensor 11 Power in Watts A constant fluid velocity profile is assumed across the gap between sensor 11 and heat sink 25. The heat quantity Q1 contained in the incremental volume 27 is proportional to the heat capacity C of the fluid, the mass (DAdz) of the fluid, and the temperature T as shown in the following equation.

Q1 = CDTAdz The heat storage rate in the incremental volume 27 is the value obtained by subtracting the rate at which heat is removed from the element by the fluid flowing at the velocity V from dQ1 / Dt, and can be expressed by the following equation.

dQ1 / dt = CDAdz (dT / dt) -CDAdz (T-T a) V = CDAdz [dT / dt- (T-T a) V] The rate of heat flow or heat flux from the sensor 11 to increment volume 27 ,
It is proportional to the area of the surface A, the thermal conductivity K of the fluid, and the vertical gradient dT / dz to the outside of the temperature, and can be expressed by the following equation.

dQ2 / dt = -KAdT / dz The rate of heat flow out of the incremental volume 27 can be expressed by the following equation.

dQ3 / dt = dQ2 / dt + d / dz (dQ2 / dt) dz = -KAdT / dz-d / dz (KAdT / dz) dz Due to heat storage, dQ2 / dt-dQ3 / dt = dQ1 / dt Therefore, -KAdT / dz + KAdT / dz + d / dz (KAdT / dz) dz = CDAdz [dT / dt− (T−T _a ) V] Therefore, d ² T / dz ² = CD / K [dT / dt− (T−T _a ). V] In the steady state, dT / dt = 0, so d ² T / dz ² = CDV / K (−T + T _a ). This differential equation, together with the following boundary condition, gives the effective volume between the sensor 11 and the heat sink 25. Uniquely describe the thermal environment within 28.

(1) At z = 0 (sensor 11), (a) dT / dz =-(1 / KA) (power supplied to the sensor 11) = -W / KA (b) T = T ₀ (2) z = G (heat sink 25 surface), T = T _a Thus, the steady state equation describing the temperature for any position z in the effective volume 28 is The steady state equation describing the temperature of the sensor at z = 0 is Therefore, the temperature rise of the sensor 11 beyond the atmosphere TR = T ₀
−T _a is expressed as follows.

In this case, constant 1 and constant 2 are determined only by the characteristics of the fluid. For the units of measurement chosen in this analysis, constant 1 = 482.4 / K, constant 2 = 103,000 (CD / K).

This equation shows that the overall variation and sensitivity of TR (temperature rise of sensor 11 above ambient temperature of flowing fluid) with respect to fluid velocity, for a desired fluid type or velocity range, simply by specifying the dimension G of the gap. It has a remarkable property that it can be specified by.

Instead of the preferred etched sensor 11 above
A prototype sensor was used in the examples described below. It has a coil of 60 turns of an annealed 0.0018 inch diameter copper wire sandwiched between two brass disks.

Example 1: P = 0.4 watts G = 0.025 inches (about 0.635 mm) A = 0.3 square inches (about 1.94 cm ² ) Fluid type = for air at standard temperature and pressure V = 0 fpm (0 m / min) TR at 100.5 ° F (approx. 55.83
TR) at V = 20 fpm (about 6.1 m / min) is 74.5 ° F (about 4
1.4 ° C) TR at V = 5000fpm (about 1524m / min) is 0.8 ° F (about
0.44 ° C).

Example 2: P = 2 watts G = 0.04 inches (about 1.02 mm) A = 0.3 square inches (about 1.94 cm ² ) Fluid type = for water, TR at V = 0 fpm (0 m / min) is 30.99. ° F (about 17.22
TR) at V = 0.1 fpm (about 3.05 cm / min) is 21.23 ° F
(About 11.79 ° C) TR at V = 20fpm (about 6.1m / min) is 0.7 ° F (about 0.3 ° C)
9 ° C).

Both of these examples demonstrate the ability to obtain measured data at high flow rates with the use of the present invention, with the high resolution available at low flow rates for both air and water. Posture and convection errors are negligible because the average temperature rise of the fluid in the working volume of the present invention at zero velocity is very small compared to conventional heat flow meters.

The second embodiment of FIG. 2 illustrates a method of minimizing spurious heat loss through the insulation surface 13 of FIG. In FIG. 2, the sensor 29 is identical to the sensor 11 of FIG. 1 but is hung at an equal distance between the two heat sinks 31,33. Fluid flows on both sides of sensor 29. The surfaces 31 and 33 of the heat sink have the same ambient temperature as the temperature of the flowing fluid. The area on both sides or sides of the sensor 29 is used in calculating the temperature / fluid velocity relationship. Due to the thickness of the substrate of sensor 29, substantially equal amounts of heat flow from sensor 29 in both directions.

FIG. 3 shows a third embodiment. In this embodiment, varying temperatures of the fluid entering the heat flux fluid flow meter can be compensated. In this embodiment, the sensor 35
Is a measurement or activity sensor similar to sensor 11 or sensor 29 of FIG. 1 or 2. Sensor 35 is 2
Located equidistant between the two heat sinks 39 and 41.

A second sensor 37 is equidistantly spaced between the heat sink 41 and another heat sink 43. Sensor 37 has the same structure as sensor 35, but it is a reference sensor. The reference sensor 37 has the same thermal characteristics as the activity sensor 35, but the power used to make the temperature measurement of the reference sensor 37 is set to be less than one-tenth of the power used in the activity sensor 35. It in this case,
DT is a value obtained by subtracting the temperature of the reference sensor 37 from the temperature of the activity sensor 35.

Still referring to FIG. 3, the battery 45 or DC power supply has a positive lead connected to one side of the coil of the sensors 35, 37. The activity sensor 35 has the other end of the coil connected to a resistor 47. In one embodiment, resistor 47 is a 10 ohm resistor. Reference sensor
37 is connected on the other side to a resistor 49. In one embodiment, resistor 49 is a 200 ohm resistor.

The opposite side of resistors 47, 49 is connected to the negative side of power supply 45. The negative side of power supply 45 is also connected to terminal C1 of a conventional analog to digital voltage data acquisition system or converter 51. The terminal C2 of the A / D converter 51 is connected between the resistor 47 and the activity sensor 35. Terminal C3 is battery
Connected to the positive side of 45. The terminal C4 is connected between the resistor 49 and the reference sensor 37. The A / D converter 51 is connected to the conventional computer 53. A / D converter 51
Collects analog voltages at terminals C1, C2, C3 and C4 and provides digital data to computer 53 for speed calculation.

In the embodiment of FIG. 3, the gap between the heat sinks 39 and 41 and between the heat sinks 41 and 43 is 0.02.
Selected to be 5 inches (about 0.635 mm). Resistor
Values of 47 and 49 are about 0.4 watts for activity sensor 35,
It produces about a hundredth of that value for the reference sensor 37. The following equations are programmed into computer 53 to produce the desired quantity.

Resistance value of the activity sensor 35: _Ra = 10 (C3-C2) / (C2-C1) [ohm] Electric power delivered to the activity sensor 35: W = (C3-C2) (C2-C1) / 10 [watt] Resistance value of reference sensor 37: R _r = 200 (C3-C4) / (C4-C1) [ohm] Temperature above ambient temperature of activity sensor 35: DT _a = 458.01 (R _a when power is supplied) -R _{a at} ambient temperature)
/ (R _a in ambient temperature) [° F] reference sensor 37 of the ambient temperature than the above temperature: DT _a = 458.01 _{(R r} when powered - R _r at ambient temperature)
/ (R _{r at} ambient temperature) [° F] Temperature rise: TR = DT _a −DT _r [° F] Generally, the formula for the apparent fluid velocity is V = (K / 103000CD) [LOG (482.4gw / AKTR ) / G] ²

Use the following values for the thermal properties of air at 70 ° F (about 21.1 ° C) and 1 atmosphere.

C = 0.24BTU / 1bF D = 0.00004641b / cu in K = 0.16BTU in / hr sq ft And the design constant selected for realizing the present invention as a fluid velocity meter is A = 0.4 sq.in (about 2.58 cm ² ) G = 0.025 in (about 0.635 mm) The specific apparent fluid velocity equation programmed is V = 2230 [LOG (188W / TR)] ² .

The resulting output speed is 2 feet per minute.
Linear within 5 percent for inputs ranging from 61m / min) to 2000 feet per minute (about 610m / min),
And the zero stability and attitude error for this embodiment of the invention is plus or minus 0.5 fpm (about 0.152 / ° C) over an ambient temperature range of 70-110 ° F (about 21.1-43.3 ° C).
Less than a minute). Even better linearity is the algorithm described above to correct factors such as the measurable series and shunt thermal impedance of the activity sensor 35 along with the deviation from the assumed constant velocity profile of the fluid due to viscosity or other factors. Can be obtained by further refining.

The present invention has important advantages. The thermal mass flow meter of the present invention improves on the high and low speed limits of conventional mass flow meters by using a unique heat flux modulation technique that allows for a wide range of fluid types and speed adaptations.

Although the present invention has been shown in three forms, it will be apparent to those skilled in the art that the present invention is not limited thereto and that various modifications can be made without departing from the scope of the present invention. Ah

The main features and aspects of the present invention are as follows.

1. a mass flow meter that measures the velocity of a flowing fluid, is suitable for being placed in a flowing fluid and is capable of producing heat when energized, and a direction substantially perpendicular to the flowing fluid Power to the sensor to produce a heat sink suitable for being placed in the flowing fluid at a selected distance from the sensor, and a heat flux that is modulated by the fluid flowing from and flowing from the sensor to the heat sink A mass flowmeter comprising a combination of means and means for measuring the temperature increase of the sensor over the ambient temperature of the flowing fluid and calculating the velocity of the flowing fluid from the measured value.

2. The flowmeter of claim 1 wherein the sensor and heat sink have opposing surfaces that are parallel to each other when positioned in the flowing fluid.

3. The flowmeter of claim 1 wherein the sensor and heat sink have opposing flat surfaces that are parallel to each other when positioned in the flowing fluid.

4. The flowmeter of claim 1, wherein the sensor comprises a coil, the sensor further having a flat surface facing the heat sink when positioned in the flowing fluid.

5. The sensor comprises a substrate laminate having a metal layer etched to provide a coil, wherein the coil has a flat surface facing a heat sink when positioned in a flowing fluid. Total.

6. A mass flowmeter for measuring the velocity of a flowing body, between a pair of metal heatsinks fitted to the flowmeter and suitable for being positioned substantially perpendicular to the flowing fluid, between the heatsinks. Held equidistant by a flow meter,
A sensor that has radiating surfaces on each side for radiating heat in opposite directions towards the heat sink and can generate heat at the radiating surface when powered, and from the radiating surface of the sensor to the heat sink. A means for powering the sensor to produce a heat flux that is modulated by the flowing and flowing fluid, and measuring the temperature increase of the radiating surface of the sensor above the ambient temperature of the flowing fluid, and based on that temperature measurement A mass flow meter comprising a combination with means for calculating the velocity of a flowing fluid.

7. A flowmeter according to claim 6, wherein the radiating surface is flat and the heat sink has a flat surface parallel to the radiating surface.

8. The flowmeter according to claim 6, wherein the sensor comprises a substrate coated with a metal layer etched to form a coil.

9. A mass flowmeter for measuring the velocity of a flowing body, a reference sensor and an activity sensor, each sensor being suitable for being placed in a flowing fluid,
Each sensor has a flat radiating surface on each side to radiate heat in opposite directions, the sensor is capable of producing heat at the radiating surface when energized and the sensor flows. Three metal heat sinks spaced apart from each other in a direction substantially perpendicular to the fluid, the emitting surfaces being parallel to each other, carrying one of the sensors, wherein one of the heat sinks is located between the two sensors, Another heatsink is located on the opposite side of the reference sensor, and yet another heatsink is located on the opposite side of the activity sensor, the heatsinks are all parallel and from one of the emitting surfaces in a direction perpendicular to the direction of the flowing fluid. Fluid that flows and flows from the emitting surface of the activity sensor to the heat sinks on each side of the activity sensor, evenly spaced Means for powering the activity sensor to produce a heat flux that is modulated, and means for powering the reference sensor at a level substantially lower than the power supplied to the activity sensor; Mass flow rate in combination with means for measuring the temperature of the radiating surface of the reference sensor and the radiating surface of the reference sensor, subtracting the temperature of the reference sensor from the temperature of the activity sensor, and calculating the velocity of the flowing fluid based on the temperature difference Total.

10. A method of measuring the velocity of a flowing fluid, wherein a sensor capable of producing heat when energized is placed in the flowing fluid and selected from the sensors in a direction substantially perpendicular to the flowing fluid. The heat sink at a fixed distance, powering the sensor to create a heat flux that flows from the sensor to the heat sink and is modulated by the flowing fluid and measures the temperature rise of the sensor above the ambient temperature of the flowing fluid. And calculating the velocity of the flowing fluid from the measured temperature.

11. The method of claim 10 further comprising disposing a second heat sink on the flowing fluid at an equidistant location on the opposite side of the sensor from the first heat sink.

12. A second sensor is disposed on the flowing fluid at a position on the opposite side of the second heat sink opposite to the first sensor, and the same distance on the opposite side of the second sensor from the second heat sink. A third heat sink is placed in the flowing fluid at a position of, and the second sensor is powered at a much lower rate than the power supplied to the first sensor, and the temperature of the second sensor is measured. , Further comprising: subtracting the measurement of the temperature of the first sensor from the measurement and using the difference to calculate the velocity of the flowing fluid.
The method described in 11.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a first embodiment of the present invention. FIG. 2 is a schematic diagram of the second embodiment of the present invention. FIG. 3 is a schematic diagram of a third embodiment of the present invention. FIG. 4 is a schematic diagram showing one of the sensors of the present invention in exploded form. 11, 29 …… Sensor 13 …… Insulation material 15 …… Substrate 17 …… Metal layer 19 …… Coil 21 …… Power supply 23 …… Measuring circuit 25,31,33 …… Heat sink 27 …… Incremental volume 28 …… Effective Volume 35 …… Activity sensor 37 …… Reference sensor 45 …… Power supply 47,49 …… Resistor 51 …… A / D converter 53 …… Computer

Claims (2)

[Claims] 1. A method comprising a sensor in contact with a flow, which emits a heat flux when an electric current flows, calculates the temperature rise in the sensor above the ambient temperature of the flow, and calculates from the temperature rise and the power supplied. A method of measuring flow velocity by using a device for calculating flow velocity, wherein the device is in contact with a fluid flow so as to receive heat flux from the sensor, the sensor (11 , 19) spaced from the sensor (11, 19) in a direction substantially perpendicular to the fluid flow. Being placed aside, the heat flux produced by the sensor (11, 19) from the sensor (11, 19) to the heat sink (25) absorbing the heat flux is substantially unimpeded, Substantially perpendicular to the fluid flow Flow, so that the heat flux is made to change by the flow of the fluid, a method that measures the rate of flow, characterized in that spacing between said sensor and said heat sink is selected. 2. A reference sensor (37) and an activity sensor (35) each adapted to be placed in a fluid flow.
Wherein the sensor (37,35) is capable of producing a heat flux when energized, the sensor (37,3)
5) reference sensors (37) spaced apart from each other in a direction substantially perpendicular to the direction of fluid flow
And an activity sensor (35), and b) three heat sinks (39,41,43), one heat sink (41) being located between the sensors (37,35) and the other one. One heat sink (43) is located on the opposite side of the reference sensor (37), and a third heat sink (39) is located on the opposite side of the activity sensor (35) and the heat sink (39,41,43). Each is substantially the same distance from an adjacent sensor (37 or 35) in a direction substantially perpendicular to the direction of fluid flow,
The heat flux generated by the activity sensor (35) cooperates with the heat sink (41, 39) from the activity sensor (35).
A heat sink cooperating with the activity sensor (35) such that the flow is substantially undisturbed by the flow of the fluid and is perpendicular to the direction of flow of the fluid and is altered by the flow of the fluid. Heat used in the method according to claim 1, characterized in that the distance to (41,39) comprises three selected heat sinks (39,41,43). Bundle measuring instrument.